The present invention relates to a control rod driving system for adjusting the power of a nuclear reactor and of a boiling water reactor (BWR) in particular.
Running operation of a reactor is basically performed by adjusting a reactivity, and a reactor power plant can be entirely controlled by suitably controlling or adjusting the degree of the reactivity. Such control of the reactivity is mainly performed by inserting or withdrawing the control rod(s), generally composed of a neutron absorber, into or from a reactor core.
Particularly, in a BWR, a core is composed of one unit consisting of a cross-shaped control rod and four fuel assemblies disposed around blade portions of the cross shaped control rod, and in this sense, the control rod can be called control blade or control blades.
The reactivity is controlled or adjusted by inserting or withdrawing such a control rod into or from the reactor core, and such insertion or withdrawal of the control rod is carried out by a control rod driving mechanism through a connecting tube.
A conventional technology in this art field will be described hereunder with reference to FIGS. 40 to 43.
Referring to FIG. 40 showing an arrangement of a control rod driving mechanism 10 disposed in a reactor pressure vessel 1, the control rod driving mechanism 10 is disposed below a core 2 and a control rod 3 is inserted into the core 2 from the lower side thereof. In FIG. 40, a lefthand control rod 3 is now inserted into the core 2 and a righthand control rod 3 is now withdrawn from the core 2. The axial length of the control rod 3 is determined to be substantially equal to a height of the core 2 and is about 4 m (meters). The moving stroke of the control rod 3 is also about 4 m, and in order to ensure the stroke, the control rod driving mechanism 10 and a connection tube 11 each have a vertical length of about 4 m. Accordingly, the entire vertical length of the control rod 3 and the control rod driving mechanism 10 is about 12 m in an operational installation.
FIG. 41 is a schematic elevational section of a conventional control rod driving system for the BWR. In FIG. 41, the control rod driving mechanism 10 is disposed in a housing 4 constructed integrally with the reactor pressure vessel 1 by welding means, for example. A control rod guide tube 5 is mounted to an upper portion of the housing 4 and the control rod 3 is accommodated in the guide tube 5. The connection tube 11 of the control rod driving mechanism 10 is connected to the control rod 3 through a coupling member 12. A driving piston 13 is mounted to the lower end of the connection tube 11, and the piston 13 constitutes a piston-cylinder assembly together with a piston tube 14 and a cylinder tube 15.
When hydraulic pressure is applied to an insertion duct 16 disposed to a lower portion of the housing 4, a driving water is introduced through the cylinder tube 15 and the piston tube 14 and then forces upward the lower surface of the driving piston 13, so that the connection tube 11 moves upward and the control rod 3 is thereby inserted into the core. The inserted control rod 3 is secured in its vertical position by engaging a collect finger 18 with a groove 17 formed on the outer surface of the connection tube 11.
When the control rod 3 is withdrawn from the core, a hydraulic pressure is applied to a withdrawal duct 19 mounted to the lower portion of the housing 4. AT this time, the driving water flows through the piston tube 14 and then into a portion between the piston tube 14 and the connecting tube 11 through a hole 20 formed in the upper portion of the piston tube 14, and finally presses downward the lower surface of the driving piston. A portion of the driving water passes between an outer tube 21 and the cylinder tube 15 to act on a collet piston 22 to thereby release the engagement between the collet finger 18 and the groove 17, whereby the control rod is withdrawn.
Mounting or dismounting of the control rod driving mechanism 10 to or from the reactor pressure vessel 1 will be described hereunder.
The control rod driving mechanism 10 is incorporated into the housing 4, from the lower side thereof, penetrating the reactor pressure vessel 1, and is secured to the reactor pressure vessel 1 by fastening the flanged portion 23 of the control rod driving mechanism 10 together with a flanged portion 6 of the housing 4 by means of bolts, for example. Accordingly, the mounting or dismounting of the control rod driving mechanism 10 can be carried out from the lower side of the reactor pressure vessel 1. Because of this arrangement, it is required to secure a wide space below the reactor pressure vessel 1 for downwardly withdrawing the control rod driving mechanism 10 having a length of about 4 m.
FIG. 42 is an illustration of the control rod guide tube 5 and elements or members associated with the guide tube 5. The control rod guide tube 5 acts to guide the vertical movement of the control rod 3 in an installed state and to support the weight of the fuel assembly 8 through a fuel support fitting 7. The control rod guide tube 5 is supported vertically at the upper end portion of the housing 4 and supported horizontally through a core support plate 9.
The control rod guide tube 5 is secured at its lower end in engagement with the top portion of a thermal sleeve 110, and this engagement is performed by relatively rotating the control rod guide tube 5 and the thermal sleeve 110. After the engagement therebetween, the control rod guide tube 5 and the thermal sleeve 110 are both fastened to prevent them from rotating to thereby completely couple them together. This fastening is done, as shown in FIG. 43, by means of a grooved lug 111 and a positioning pin 112 provided on the core support plate 9. The mounting or dismounting of the control rod guide tube 5 is performed from the upper side of the reactor pressure vessel 1.
As described above, the conventional control rod driving system including the control rod driving mechanism has a long vertical length of about 12 m in an extended condition and about 8 m even in a contracted condition. For this reason, in the conventional arrangement of a nuclear plant, the major portion of the control rod driving mechanism is disposed outside the reactor pressure vessel in the manner of penetrating the bottom thereof and without completely accommodating the mechanism into the reactor pressure vessel. Accordingly, when the control rod or the control rod driving mechanism is mounted to or dismounted from the lower side of the reactor pressure vessel, a large space is required for it, resulting in enlargement of a reactor building itself, which involves an increased construction cost. Moreover, in such an arrangement, the reactor pressure vessel is installed at a relatively high position, thus being disadvantageous for seismic resistance, for example.
Furthermore, the formation of the penetrating portion in the bottom of the reactor pressure vessel will provide problems such as the coolant in the reactor may leak externally therethrough, which is dangerous for persons who work at the lower portion of the reactor containment vessel when mounting or dismounting the control rod driving mechanism.
Still furthermore, it is necessary to exchange the piston cylinder mechanism periodically due to wear of the piston ring of the assembly, and with respect to this, there is also a problem for maintenance because such exchanging work requires the withdrawal of the control rod driving mechanism from the lower side of the reactor pressure vessel and the disassembly of the same.